1. Field of the Invention
The present invention relates to a printing medium suitable for use in printing with water-based inks. In particular, the present invention relates to a printing medium, which can prevent the occurrence of beading and provide images high in optical density, bright in color tone and high in resolution, and has excellent ink-absorbing capacity.
The present invention also relates to a printing medium has good surface hardness and film properties in addition to the above-described characteristics or properties.
The present invention further relates to a production process of the printing medium and an image-forming process using this medium.
2. Related Background Art
In recent years, an ink-jet recording system, in which minute droplets of an ink are flown by any one of various working principles to apply them to a printing medium such as paper, thereby making a record of images, characters and/or the like, has been quickly spread as a recording apparatus for various images in various applications including information instruments because it has features that printing can be conducted at high speed and with a low noise, color images can be formed with ease, printing patterns are very flexible, and development and fixing process are unnecessary. Further, it begins to be applied to a field of recording of full-color images because images formed by a multi-color ink-jet system are comparable in quality with multi-color prints by a plate making system and photoprints by a color photographic system, and such printed images can be obtained at lower cost than the usual multi-color prints and photoprints when the number of copies is small. With the improvement in printability such as speeding up and high definition of printing, and full-coloring of images, printing apparatus and printing methods have been improved, and printing media have also been required to have higher properties.
In recent years, printing media having a coating layer, in which an alumina hydrate of a boehmite structure is used, have also been proposed and are disclosed in, for example, U.S. Pat. Nos. 4,879,166 and 5,104,730, and Japanese Patent Application Laid-Open Nos. 2-276670, 4-37576 and 5-32037.
The printing media using these alumina hydrates have advantages that since the alumina hydrates have a positive charge, a dye in an ink is well fixed, that since an ink-receiving layer comprising such an alumina hydrate has good transparency, an image high in optical density and good in coloring can be provided, that problems such as bronzing of a black ink and reduction of light fastness, which have heretofore been caused by the use of silica compounds, are not caused and moreover that they are better than the conventional printing media from the viewpoint of the quality and gloss of images formed thereon, in particular, full-color images, and application to sheets for OHP.
In order to have a printing medium fully exhibit the advantages inherent in these alumina hydrates, it is however necessary to improve the following respects:
1) Since the quantity of inks ejected on a printing medium is increased for coping with the demand for the provision of a high-quality image upon, in particular, full-color printing in recent years, the printing medium is required to have high ink absorbency. If the printing medium has poor ink absorbency, inks ejected are more than its ink-receiving layer can absorb them in pores thereof and hence run out of the surface of the ink-receiving layer, so that there are involved problems that bleeding occurs to deteriorate print quality and color reproducibility, and that the fluid inks before fixing run and aggregate to cause beading (unevenness of printing).
The term "beading" as used herein refers to a phenomenon caused by the fact that droplets of inks applied to a printing medium aggregate into larger droplets in the course of absorption and/or the like. It is said that the beading is easy to occur on media low in ink absorbency or slow in fixing speed of a dye in an ink, or even on media having good ink absorbency when inks are applied in plenty.
In a printing medium provided with an ink-receiving layer, the beading is observed on the surface of the ink-receiving layer and also in the interior of the ink-receiving layer when it is transparent.
In order to solve such a problem of beading, it is disclosed in Japanese Patent Application Laid-Open No. 1-222985 to apply hydrophobic fine particles such as oil droplets or particles of a fluorocarbon resin to the surface of an ink-receiving layer.
According to this method, however, it is not easy to adjust the amount of the hydrophobic fine particles to be applied. If the amount thereof is too little, the effect of the application cannot be fully exhibited. If the amount is too great on the other hand, the ink absorbency of the resulting printing medium becomes deteriorated, and so it rather becomes easy to induce beading.
Besides, the effect of this method on the beading is limited to the surface of the ink-receiving layer and is scarcely exhibited on beading in the interior of the ink-receiving layer, which is caused by ink penetrated into the ink-receiving layer.
It is disclosed in Japanese Patent Application Laid-Open No. 60-224580 to use synthetic amorphous silica, the surface of which has been treated with a silane coupling agent, in an ink-receiving layer, thereby adjusting the degree of feathering of dots. It is also disclosed in Japanese Patent Application Laid-Open No. 62-178384 to use silica, the surface of which has been treated with a silane coupling agent, in an ink-receiving layer, thereby improving the light fastness of an image to be formed.
However, silica is large in size of secondary aggregate, and so a porous ink-receiving layer containing this silica has a mat and white coating surface. Therefore, such a sheet can be applied only to coated paper using paper as a substrate and not applied to a medium which can provide a high-gloss image like a photoprint, which meets the demand for formation of a high-quality image in recent years, or to a transparent sheet for OHP.